We are using resonance Raman spectroscopy with UV excitation to study tertiary and quaternary interactions in Hb, via the enhanced vibrational signals of aromatic side chains. Indicators of specific molecular contacts have been established, and they are being used to monitor the time evolution of structure change using time-resolved pulse-probe techniques. We are now turning to Hb's which are modified at specific sites by genetic or chemical methods. These modified samples require characterization to assure integrity of the modification and sample homogeneity. Gel techniques are inadequate because of limited sensitivity to single residue deletion or modification in a relatively large protein. The mass spectrum of the purified protein would provide the most reliable characterization.